1. Field of the Invention
The present invention relates to a new, very high frame rate, read out chip designed for single photon counting in combination with a separate semiconductor material for the photoelectric effect. This chip is the basic module for new detector systems for X-ray applications at synchrotrons or with lab equipment (lab diffractometers): material science, crystallography, non destructive testing and medical applications; energy range: 1-200 keV.
2. Current State of the Art (for Single Photon Counting)
Hybrid pixel detectors consist of a pixilated X-ray sensitive layer (silicon sensor) and a corresponding pixilated readout chip. Each pixel in the sensor is directly connected (bump bonding or flip chip bonding) to the corresponding pixel in the readout chip. A generic pixel detector is disclosed in the European Patent Application 1 581 971 A1.
The readout chip contains an array of n×m independently working channels (pixels). Each channel has a charge sensitive preamplifier with tunable gain, a signal shaper with tunable shaping time, a comparator and a counter with simple pixel control and readout logic. A photon impinging a sensor pixel generates electron-hole pairs. These electron-hole pairs are separated by an electric field generating a charge pulse. This charge signal from the sensor is amplified and filtered by the low noise preamplifier and shaper in the corresponding pixel cell (in the readout chip). The shaped signal is fed to a comparator with a global reference voltage and an on-pixel trim DAC. An incoming signal exceeding this threshold will toggle the comparator state. If the chip is in Expose mode (counting the photons), the comparator pulse increments the digital counter by one. During the Readout phase the pixel counter states are serially transferred to the chip periphery, where they are readout via dedicated readout logic.
The known state of the art according to the EP 1 581 971 A1 has several limitations for single photon counting:
i) A general problem of readout pixel chips is related to the pixel size. Known pixel detector have a pixel size in the range of about 172 μm2. A smaller pixel size can increase the image resolution, but limits also the number of transistors, and hence the functionality, that can be put on the pixel itself. This is specifically the case for radiation hard designs where the transistors are larger as compared to standard transistors.
ii) For fast frame rates the readout time (dead time) is very significant and limits the frame rate. Many measurements are currently limited by the frame rate. The pixel detector known in the prior art having 256×256 pixels at a data depth of 12 bit requires at a read-out rate of 200 MHz about 4 to 6 ms for its readout.
iii) For pump and probe measurements a sample is excited (pumped) and then after a selectable time the counting is enabled for a short period (probe). This is then repeated as often as required to gain statistics, accumulating the images. Where the conditions are not constant it is necessary to make (at least) 2 simultaneous measurements (usually pumped and un-pumped). This is currently not possible since the counts can only be accumulated in one internal counter.